There have been many inventions in the field of the cellular communication systems and fourth generation communication technologies for in the last few years. The network MIMO systems referred to as multi-cell MIMO systems are focused on the IMT-Advanced and beyond systems. In such multi-cell MIMO networks, the inter-cell interference is one of the significant problems. In the well-designed multi-cell MIMO systems, the inter-cell coordination can be accomplished through cooperative signal configuration. There are many methods well-known for making in possible to reach certain interference channel capacity through efficient cooperative multi-cell processing design. One of the well-known methods is interference alignment.
The interference alignment is a cooperative processing of the transmitters/receivers. This is to generate overlapped signal spaces occupied by unwanted interferences at the respective receivers while maintain the wanted signal spaces. On the time or frequency varying channel, the interference alignment is capable of achieving symbol extension in time and frequency domain. In the present disclosure, the main focus is on the space rather than time and frequency. The spatial freedom degree is conceived along with constant channel coefficient.
The channel alignment for three users (K=3) interference channel having multiple antennas (M>1) is known to have the external and internal boundaries of freedom degree that are equal to each other. When the external and internal boundaries of the freedom degree match each other, it is regarded that the interference alignment has achieved the optimal freedom degree. However, it has not been known whether such a result can be extended to the case of normal number of users (K>3) and plural receive and transmit antennas. In the different networks such as X networks and cellular networks, the constant channel coefficient of the interference alignment for achieving the optimal freedom degree is not known yet.